rigid

Describe something as rigid if it's really stiff and not very flexible, like a super-strict practice schedule or an unbending, uncomfortable mattress.

The adjective rigid is actually related to the Latin word frigus, which means “cold.” So think of a rigid icicle or a rock-hard igloo to help you remember what rigid means. You can also use it to describe something that's particularly strict or unable to change, like the stubborn views of a person who just won't consider another perspective.